1. Field of the Invention
The present invention concerns a device and a method for determining printing-related measured values from a gradient patch printed as a control patch and a corresponding computer program.
2. Description of the Related Art
In the control of printing processes, it is necessary to determine quality-relevant parameters as often as possible in order to manage the printing process according to the determined parameters. Quality-relevant parameters are all parameters that can describe the reproduction of the subject on the medium to be printed, e.g., paper. In particular, it is important to check the reproduction of chromatic and achromatic inks. Examples of quality-relevant parameters are therefore the degree of area coverage or screen tone value that has been achieved, the tone value increase, or the full-tone density achieved with an ink. To determine these parameters, in industrial printing, e.g., in offset printing with a rotary press, print control strips are printed during the printing operation along with the actual printing job. To determine some quality-relevant parameters, for example, the screen tone value or the full-tone density of an ink, control patches are printed by the printing press as part of print control strips.
A control patch is an area printed on the printing medium and contains an image area. In order, for example, in the prior art, to check the screen tone value, several control patches are arranged in a row to form a control patch strip, and the control patches represent a possibly different percentage of area coverage. Due to limited space, as few control patches as possible are usually printed on the prior-art print control strip. For example, the use of three control patches in the form of a tone value wedge with a degree of area coverage of 25%, 50%, and 75% is known. Test charts for determining the tone value increase generally consist of these tone value wedges reproduced in the given process color. However, the test charts known from the prior art allow only imprecise measurement of the tone value increase. In accordance with the prior art, to obtain a more precise measurement over the entire tone value range, the space requirement that would be necessary for exact determination of the tone value increase would increase considerably.
On the basis of the evaluation of the quality-relevant parameters, e.g., the screen tone value or the full-tone density of an ink, the technical personnel can carry out adjustment steps on the printing press to optimize the printing result. These adjustment steps can be automatically carried out in the same way by a suitable device. Customary adjustment steps on an offset press involve metering of the ink and water.
The relationship between quality-relevant parameters and adjustment steps is explained below on the basis of the example of the full-tone density of an ink. The full-tone density of an ink is measured by densitometry, i.e., the optical density of an ink is determined in a control patch. The optical density is an important characteristic in the printing process, which is directly related (almost linearly in certain subdomains) to the ink layer thicknesses transferred in the printing process, as FIG. 1 shows. Measurement of the optical density and control of the printing process on the basis of the density values represents a possible means, widely used in the prior art, of maintaining the ink metering constant in the printing process.
Some measurement data of quality-relevant parameters, e.g., the tone value increase, are fed back to the prepress phase in order to achieve control of the printing process. The measurement data are processed in the prepress phase, e.g., into correction curves or an ICC (International Color Consortium) color profile, i.e., generally speaking, they are used in computational steps of the prepress phase.
To determine the parameters for preparing the ICC color profile, the space requirement is considerable for printing technology, since test charts for preparing an ICC color profile normally have a large number of control patches, including not only control patches with the given primary colors but also a great variety of control patches with mixed colors. In this regard, it can be said that the more colors are measured, i.e., the more control patches are available, the more exactly the ICC color profile can be prepared. However, to realize the best possible utilization of the printing press, it is necessary to cover the largest possible portion of the printed area with the actual printing job. The present invention arose from the conflicting interests that have been described, i.e., on the one hand, improved qualitative evaluation of the printing process by the largest possible number of control patches and, on the other hand, the necessity of saving printed area.